The union of two subspaces just means that we include all of the elements of each subspace into another space,obeying the same rules as they did before.Here the elements are vectors.( This just like the union of two sets.no different.)
So you have all the vectors in subspace M and all the vectors in subspace N together. Since you are told that the vector x is either in M or N,then the union of M and N must contain ALL the vectors x ,and they must be linearly independent.(Since they are linearly independent in both M AND in N)
Now ,both linear subspaces obey a(x1 + x2) = ax1 + ax2 where x1,x2 are any two linearly independent vectors in either M or N.
Finally,if x is in either M or N,it cannot be outside M or N,so you cannot find vectors x, outside M U N,hence M U N must be a linear subspace of V. (take heart,no-one finds this stuff easy to begin with.)